Background Renal dysfunction or acute renal failure in patients undergoing coronary artery bypass grafting (CABG) is one of the most important causes of morbidity and mortality. The great effect of acute renal dysfunction in the outcomes of CABG surgery demands its study in our population, encouraging to the elaboration of this study, which aimed to identify the incidence and risk factors of renal dysfunction after CABG.
Patients and methods Since January 2013 to December 2014, 290 patients were studied who underwent CABG with preoperative normal renal function. In this cross-sectional study, patients were divided into two groups based on the occurrence of renal dysfunction after CABG, and measured variables were compared between the two groups and statistically analyzed. P value less than 0.05 was set as a significant level.
Results Renal dysfunction was seen in ∼21.37% of patients after CABG. The mean age of renal dysfunction group was higher than that in the other group, and the difference was significant between the two groups. Moreover, reduced ejection fraction was significantly different between the two groups. Cardiopulmonary bypass time was also statistically significant. Postoperative hemodynamic instability and postoperative bleeding were also statistically significant.
Conclusion Our study showed older patients were more prone to acute renal failure. Conditions that affect renal perfusion as reduced ejection fraction, hemodynamic instability, and postoperative bleeding are associated with increased risk of renal dysfunction.

Renal dysfunction or acute renal failure (ARF) in patients undergoing CABG is considered as an important cause of morbidity and mortality [3].

The presence of conditions that determine hypoperfusion and renal ischemia is directly related to the development of ARF. Patients who present with reduced renal functional reserve, in whom a reduction occurs in the glomerular filtration rate without serum creatinine elevation above normal values, are more likely to have ARF even with minor renal lesions. Preoperative and intraoperative factors, such as age, previous level of creatinine, diabetes mellitus, cardiac output, the duration of extracorporeal circulation, and the use of the intra-aortic balloon, are influential in the development of ARF [2],[4],[5],[6],[7],[8],[9],[10].

A decrease in cardiac output in the early stage following cardiac surgery is an important risk factor of ARF [11].

ARF is a severe condition that occurs in 2.0–7.0% of patients during hospital stay [12],[13],[14]. In 18.0–47.0% of cases, it is related to a surgical event, and acute tubular necrosis is considered the main type of lesion [4],[12],[13]. The great variation in the incidence of ARF among the studies is owing to different factors, including different diagnostic criteria, such as the design of study, inclusion and exclusion criteria, profile of the patients, and of the centers in which the study was done, hindering study comparisons [15].

The term ARF was used until 2004 for cases when a patient’s GFR suddenly decreased. However, ARF had no standard definition at that time. More than 35 different definitions of ARF were reported in the literature, ranging from a small increase in serum creatinine to renal failure requiring dialysis [7],[16],[17].

From 1 January 2013 to 31 December 2014, 290 patients undergoing myocardial CABG surgery were retrospectively studied after exclusion of patients with preoperative renal dysfunction. After obtaining the approval of the local ethical committee.

Patients were divided into two groups based on the occurrence of renal dysfunction after CABG according to RIFLE criteria, and measured variables were compared between the two groups.

We examined the following variables as possible predictors of RENAL dysfunction: age; sex, weight; BMI; dyslipidemia; preoperative ejection fraction (assessed by two-dimensional echocardiography); history of diabetes mellitus; history of hypertension; CPB time; cross-clamp time; number of grafts; if weaning from CPB needed inotrope, cardioversion, pacemaker, or the use of the intra-aortic balloon; and postoperative data such as hypoxia, postoperative bleeding, arrhythmia during 48 h postoperatively, hemodynamic instability, extubation time postoperatively, and the need of inotrope more than 12 h postoperatively.

Statistical analysis

Data were fed to the computer and analyzed using IBM SPSS software package, version 20.0 (IBM Corp., Armonk, New York, USA). Qualitative data were described using number and percent. The Kolmogorov–Smirnov test was used to verify the normality of distribution. Quantitative data were described using range (minimum and maximum), mean, SD, and median. Significance of the obtained results was judged at the 5% level.

The used tests were χ2-test for categorical variables, to compare between different groups, Fisher’s exact correction for χ2 when more than 20% of the cells have expected count less than 5, and Student’s t-test for normally quantitative variables to compare between two studied groups.

Results

Renal dysfunction in the postoperative period occurred in 62 (21.37%) of the 290 study patients according to RIFLE criteria.

They were classified according to RIFLE criteria as follows: stage of risk − 48 patients, who represent 16.5% of the total number of the study patients and 77.4% of the patients with renal dysfunction; stage of injury − eight patients, who represents 2.75% of the total number of the study patients and 12.9% of the patients with renal dysfunction; and stage of failure − six patients, who represents 2.06% of the total number of the study patients and 9.67% of the patients with renal dysfunction.

Incidence of dialysis

Three of six patients at the stage of failure required dialysis.

Incidence of mortality

Two (0.87%) patients of 228 patients with postoperative normal renal function, two (4.16%) patients of 48 patients at stage of risk, one (12.5%) patient of eight patients at stage of injury, and two (33.3%) patients of six patients at stage of failure died ([Table 2]).{Table 2}

There was a significant difference between the two groups regarding age, BMI, and preoperative ejection fraction.

There was no significant difference between the two groups regarding weight ([Table 3]).{Table 3}

There was a significant difference between the two groups regarding the occurrence of diabetes mellitus and hypertension ([Table 4]).{Table 4}

There was a significant difference between the two groups regarding the increase in CPB time.

There was no significant difference between the two groups regarding the increase in cross-clamp time and number of grafts ([Table 5]).{Table 5}

There was no significant difference between the two groups regarding the need for intraoperative inotropes, cardioversion, pacemaker, and intra-aortic balloon ([Table 6]).{Table 6}

There was no significant difference between the two groups regarding postoperative duration of mechanical ventilation.

There was a significant difference between the two groups regarding postoperative bleeding ([Table 7]).{Table 7}

There was no significant difference between the two groups regarding postoperative hypoxia, arrhythmia, and the need for inotropes for more than 12 h.

There was a significant difference between the two groups regarding postoperative hemodynamic instability ([Table 8]).{Table 8}

Discussion

ARF is a potential complication of CABG that can arise from a variety of causes including preoperative comorbidities intraoperative hypotension, postoperative cardiac complications that impair renal perfusion, atheroemboli, and exposure to contrast media [20].

In the present study, we considered RIFLE criteria as an index of renal dysfunction.

Renal dysfunction in the postoperative period occurred in 62 (21.37%) of the 290 study patients.

They were classified according to RIFLE criteria as follows: stage of risk − 48 patients who represent 16.5% of the total number of the study patients and 77.4% of the patients with renal dysfunction; stage of injury − eight patients who represent 2.75% of the total number of the study patients and 12.9% of the patients with renal dysfunction; and stage of failure − six patients who represent 2.06% of the total number of the study patients and 9.67% of the patients with renal dysfunction.

The incidence of ARF described by Andersson et al. [10] was 16.4% and Zanardo et al. [9] was 15.1%. However, it was described by Mangano et al. [2] to be 7.7% and Conlon et al. [7] to be 8.0%, using less strict criteria for the diagnosis of ARF. According to Mangano et al. [2], the parameters of ARF are creatinine level in the postoperative period as more than 2.0 mg/dl and the increase in serum levels of creatinine more than 0.7 mg/dl above that value of the preoperative period.

According to the parameters used by Conlon et al. [7], ARF is the increase in the levels of serum creatinine more than 1.0 mg/dl above the preoperative period value.

In a review performed by Bridgewater et al. [21] from 1999 to 2002 of more than 51 000 CABG procedures, the incidence of ARF was constant over the 4 years, ranging from 4 to 5%. In a 2006 data analysis report from the STS by Santos et al. [22], the incidence of ARF was 3.6% after isolated CABG, and 7.5 and 12.9% after CABG combined with aortic or mitral valve replacement, respectively.

Since March 2010–2011, 589 patients were studied who underwent CABG by Mirmohammad-Sadeghi et al. [23]. Acute renal dysfunction was defined based on peak serum creatinine level more than 2 mg/dl or more than two-fold increase in postoperative creatinine level. ARF was defined as deterioration in renal function sufficient enough to require dialysis within 30 days following surgery. ARF was observed in ∼22% of the patients after CABG with normal preoperative renal function [23].

The incidence of mortality between patients with postoperative renal dysfunction (five of 62: 8%) was higher than that of patients with normal postoperative renal function (two of 228: 0.87%), and the incidence of mortality increase with increased severity of renal dysfunction.

According to Santos et al. [22] the mortality rate observed in patients without ARF was 1.6%, compared with 25.0% for those with it.

According to Conlon et al. [7] the mortality rate observed in patients without ARF was 1% compared with 14% for those with it.

According to Provenchère et al. [24] the in-hospital mortality rate among all patients without postoperative renal dysfunction was 1.6%. In contrast, it was 27.5% in patients with postoperative renal dysfunction.

Age is considered as a risk factor for renal dysfunction. This result is consistent with other studies that suggested that age older than 63 years was an independent risk factor, possibly owing to loss in renal functional reserve by the progressive decrease in the glomerular filtration rate, which is evidenced by age, making these patients more susceptible to more severe renal lesion because of renal hypoperfusion [4],[7],[10].

There was a significant difference between the group of postoperative renal dysfunction and the group of postoperative normal renal function regarding diabetes mellitus (P=0.006) in univariate analysis, but according to multivariate analysis, it was not significant (P=0.278).

In contradictory to these findings, Conlon et al. [7] considered diabetes mellitus to be an independent risk factor for postoperative renal dysfunction.

There was a significant difference between the group of postoperative renal dysfunction and the group of postoperative normal renal function regarding hypertension (P=0.047) in univariate analysis, but according to multivariate analysis, it was not significant (P=0.45). This result is contradictory to other studies that suggested that preoperative blood pressure elevation is known to predispose to renal dysfunction [18],[25].

Patients with preoperative left ventricular dysfunction may have reduced ability to cope with stress of complicated surgery and hemodynamic derangements that contributed to postoperative renal function deterioration [7],[16].

Increased rate of postoperative bleeding is considered as a risk factor for renal dysfunction [24]. Hypovolemia and hemodilution owing to postoperative blood loss increase the risk of renal dysfunction after surgery, probably because of the lower oxygen supply to the renal medulla. There is increasing evidence that transfused red blood cells (RBCs) for management of bleeding may actually contribute to organ injury in susceptible patients, likely because of changes that occur to RBCs during storage [30]. Transfused stored RBCs may impair tissue oxygen delivery, promote a proinflammatory state, exacerbate tissue oxidative stress, and activate leukocytes and the coagulation cascade [31],[32],[33]. In susceptible patients, such as those undergoing cardiac surgery, these changes can lead to organ dysfunction, with the kidney seemingly at particularly high risk for injury [30].

Postoperative hemodynamic instability is considered as a risk factor for renal dysfunction [34].

Conclusion

Our data showed older patients were more prone to be affected by renal dysfunction.

Renal dysfunction was found to increase with the increased CPB time.

Factors that lead to hemodynamic instability such as reduced ejection fraction and postoperative bleeding are considered as risk factors for renal dysfunction.